1. Field of the Invention
This invention relates to ultrasonic laser-based contactless inspection systems, and more particularly to such systems with two and three-dimensional imaging, electronically controllable steering, focusing and phasing capabilities and self-calibration capabilities.
2. Description of the Related Art
Ultrasonic waves are commonly used to probe a variety of materials, particularly for thickness gauging and flaw detection. The sound waves have usually been generated with a contact piezoelectric transducer (PZT). The launched waves propagate through the material, reflecting from interfaces (in thickness gauging applications) or internal features (in flaw detection applications). The scattered sound propagates back to the surface of the workpiece, causing the surface to vibrate at the ultrasound frequency. This vibration is usually detected with a contact PZT similar to the one used to generate the sound.
Optical detection techniques, such as those described in C. B. Scruby and L. E. Drain, Laser Ultrasonics, Techniques and Applications, Adam Hilger, New York (1990), pages 325-350, can be used in place of the piezoelectric transducers to remotely detect the workpiece vibrations. Generally, a laser probe beam is directed onto the workpiece. When the surface vibrates it imparts a phase shift onto the reflected beam. This phase shift is detected with a photodetector after mixing the reflected probe beam with a stable reference beam and measuring the amplitude and frequency or phase of the detector output intensity fluctuations. The reference beam originates from the same laser source as the reflected probe beam, and the output signal from the photodetector or an electronic phase detector corresponds to the surface motion.
One problem with laser detection systems is that extraneous mechanical noise sources can cause additional low frequency vibrations at the surface. These additional vibrations are picked up by the reflected probe beam and reduce the signal-to-noise ratio of the detected signal.
Another problem is low sensitivity. Typically, the workpiece surface that is being probed has a diffusely reflecting or scattering quality. Consequently, the reflected beam is highly aberrated and its wavefront is mismatched with respect to the reference beam. The aberrated reflected beam produces a "specklet" field distribution on the optical detector that is used to detect the optical interference between the reflected and reference beams. The phase relationship between the reflected probe beam and the reference beam is maintained only over a single "speckle" diameter. Consequently, the phase relationship can be set optimally only for light within the speckle area; light within other speckles will have a different and generally nonoptimal phase relationship with the reference beam. The resulting detector signal can thus be thousands of time weaker, due to multiple speckle capture, than would be the case if the surface were a perfect mirror (in which all light would be in a single speckle).
One prior laser-based ultrasonic detection system, described in U.S. Pat. No. 5,131,748 entitled "Broadband Optical Detection of Transient Motion From a Scattering Surface by Two-Wave Mixing in a Photorefractive Crystal", issued Jul. 21, 1992 to Jean-Pierre Monchalin, et al., addresses the wavefront matching problem. In this system the reflected probe beam optically interferes inside a photorefractive crystal with a "pump" beam that is derived from the same laser. The two beams form a refraction grating inside the crystal that diffracts the pump beam in the propagation direction of the probe beam so that the beams overlap and have substantially matching wavefronts when they exit the crystal. However, the grating matches the phases of the diffracted pump beam and the probe beam, which causes the system's sensitivity to the surface vibrations to be very small in the case of a homodyne detection system. To overcome this problem, a second frequency shifted pump beam is superimposed onto the first pump beam, permitting heterodyne detection. The second pump beam is close enough in frequency to the first pump beam to be Bragg matched to the index grating and, therefore, diffracts off this grating. A second index grating is not written by the second pump beam and the probe beam because the crystal cannot respond fast enough to the moving fringe grating produced between the beams (the frequency shift between the beams results in non-stationary fringes). As a result, the second pump beam only diffracts off the first stationary grating (written by the first pump beam and the probe beam), and the relative phase between it and the probe beam is preserved.
Although this technique improves the system's sensitivity, it suffers from many limitations. Although the wavefronts of the probe beam and the diffracted pump beam are matched, they are matched to the aberrated and highly speckled wavefront of the probe beam rather than to the clean wavefront of the pump beam, resulting in a degradation of the coherently detected beam. Second, if the workpiece surface is de-polarizing, the sensitivity of the detector goes down. In addition, if the workpiece surface contains highly contrasting features (for example, pits, rust, spots, etc.), the two-wave mixing amplification may result in non-uniform "print-through" (due to pump depletion), which degrates the system performance. Finally, the Monchalin system does not automatically compensate for extraneous acoustic noise sources which cause additional vibrations at the surface. These spurious noise sources can dominate the signal limit of the desired feature to be detected and, moreover, can lie in the same ultrasonic frequency band, thereby obscuring the sending of the desired signal. Also, a single detection spot will limit the spatial resolution of the system, so the precision mapping of the desired feature's location may not be resolvable. Furthermore, the use of a single detection spot will place a limit on the sensitivity of the diagnostic, since beyond the threshold probe intensity, optical damage to the sample can result.
An alternative interferometric technique for detecting ultrasound uses a "self-referencing" interferometer that produces an output proportional to a temporal difference vibration signal, rather than to the displacement, of the moving workpiece surface. Time-delay interferometry, described in the Scruby et al. book, pages 123-127, is one such technique. In time-delay interferometry the probe beam that is reflected from the workpiece surface is split into two interferometer beams and then recombined at a standard photodetector, with one of the beams time-delayed with respect to the other such as by having it traverse a longer distance. The two beams are collinear when they are recombined at the photodetector, and the light intensity at the photodetector is proportional to the velocity of the workpiece surface. Ideally, the reflected readout beam is interfered with a time-delayed replica of itself and the wavefronts of the two interfering beams are substantially matched. Consequently, a phase shift in one leg of the interferometer is common to all speckles, and all speckles can be detected optimally. Unlike a conventional interferometer, which has a flat frequency response to phase shifts, a time-delay interferometer has a band-pass type of response. The time-delay interferometer suppresses both the low frequency (below ultrasonic frequencies), as well as certain ultrasonic frequency vibrations.
With this type of system, however, the speckles are often so plentiful and small as to not be compatible with each other after one arm of the interferometer has been time-delayed. The time delay is most easily accomplished with a multi-mode fiber, which further increases the number of speckles and scrambles their locations, making speckle registration impossible. Thus, bulky, long-path-length and expensing precision "re-imaging" interferometer such as a Fobery-Perot unit, is employed. (Furthermore, the time delay must be held constant and stabilized to hold the optical beams in quadrature, if homodyne detection is employed. If the path length difference that causes the time delay is not maintained to within a fraction of a wavelength, the sensitivity of a homodyne system will be greatly reduced.) As a result, velocity interferometers must typically employ active stabilization techniques. In industrial environments, the frequency range and amplitude of the noise-induced vibrations reduce the effectiveness of active stabilization techniques.
In pending application Ser. No. 08/404,660, filed Mar. 15, 1995 for "Laser-Ultrasonic Non-Destructive, Non-Contacting Inspection System" by the present inventors and others, a phased-array contactless optical excitation and detection scheme is disclosed in which an array of acoustic waves are generated in the workpiece by a short pulse optical transmitter beam with a beam geometry that is tailored to focus the acoustic waves at an inspection site within the workpiece. The acoustic waves are then detected by reflecting an optical readout beam from a vibrating surface of the workpiece and optically interfering it with a reference beam. The readout beam geometry causes it to detect only those acoustic waves that arrive from the focal inspection site; other acoustic waves are out of phase with each other and cancel. The system employs relatively expensive and complex heterodyne detectors with post-processing electronic tracking hardware to compensate for large amplitude, low-frequency whole-body motions typical in an industrial environment. It uses optical summation for beam formation and thus requires path compensation via photorefractive wavefront compensators, which are relatively slow and introduce occasional signal dropouts.
Another pending application by the present inventors and others, Ser. No. 08/481,673, filed Jun. 7, 1995 for "A System and Method for Detecting Ultrasound Using Time-Delay Interferometry", discloses another contactless system in which an optical probe beam is again reflected and phase modulated by a workpiece surface that is vibrated by ultrasound. A time-delay interferometer optically interferes the phase modulated probe beam reflection with a time-delayed replica of itself to produce interference fringes that move in accordance with the workpiece surface displacement temporal differences. The fringes are detected by a non-steady-state photo-electromotive-force (NSSPEMF) detector that generates an output signal when the frequency of the fringe motion exceeds a given threshold. While this system is relatively insensitive to rough workpiece surfaces, suppresses low frequency noise and provides high sensitivity without the need for active stabilization, the delay lines require long, cumbersome optical fibers to form the beam with attendant input-output coupling loses and power limiting non-linear effects.
NSSPEMF detectors are described in M. P. Petrov et al., "Non-steady-state photo-electromotive-force induced by dynamic gratings in partially compensated photoconductors", Journal of Applied Physics, Vol. 68, No. 5, (1990), pages 2216-2225, and in S. I. Stepanov et al., "Measuring Vibration amplitudes in the picometer range using moving light gratings in photoconductive GaAs:Cr", Optical Letters, Vol. 15, No. 21, (1990), pages 1239-1241. They are able to remotely detect minute ultrasonic vibrations (on the order of picometer to nanometer displacements over a 1-30 MHZ bandpass), while automatically compensating for static or dynamic phase noise below about 10 kHz. However, the output of this type of device does not by itself yield a quantitative measure of the workpiece's ultrasonic response--it reveals only qualitative information such as the shape of the surface displacement as a function of time. To properly interpret, classify and process the output information of an ultrasonic sensor or inspection system, quantitative ultrasonic information such as the absolute value of the surface displacement vs. time should be obtained. The output lens of the sensor is not only proportional to the desired displacement, it is also influenced by typical industrial perturbations such as intensity fluctuations caused, for example, by pits or rust, as well as scattering changes due to variations in surface roughness, which can narrow the field-of-view of the class of sensors. Without such information the sensor's output is of rather limited use, even when a non-rule-based neural network processor is employed to classify the information. Moreover, feedback control of the manufacturing process cannot proceed without correct calibration and interpretation of the data.